Performance of Supercapacitor Using Carbon Electrode Prepared via Microwave Activation of Japanese Distilled Liquor Waste
Article ID: 668
Details
Article ID: 668
ABSTRACT Japanese distilled liquor (Shochu) is a widely produced alcoholic beverage in the Kyushu region of Japan. Our research group is developing inexpensive electrode material for supercapacitors (SCs) using waste generated during shochu production. In a previous study, shochu waste was carbonized and activated in an electric furnace; however, this process involved long production times and high running costs. To address these limitations, this study investigated the use of microwave activation as a faster and more cost-effective alternative. Activated carbon (AC) was fabricated from carbonized shochu waste mixed with potassium carbonate in a 1:3 weight ratio. Microwave power was set to 800 W, and the activation time was varied. The performance of the AC samples was evaluated by analyzing surface morphology, specific surface area, and pore volume, along with measurements of specific capacitance and charge-transfer resistance. The results revealed a maximum specific surface area of 1,063 m 2 /g and a specific capacitance of 229 F/g in 0.5 mol/L KOH at an activation time of 9 minutes. Additionally, microwave activation reduced power consumption costs by approximately 61.9% compared to activation using an electric furnace. Keywords: Supercapacitor; shochu waste, activated carbon, microwave activation
Performance of Supercapacitor Using Carbon Electrode Prepared via Microwave Activation of Japanese Distilled Liquor Waste
Daisuke TASHIMA1, Akio IZAKI1, Takuya EGUCHI2, Toshiki TSUBOTA3, Seiji KUMAGAI4
1 Department of Electrical Engineering, Graduate School of Engineering, Fukuoka Institute of Technology, 3-30-1 Wajirohigashi Higashi-ku, Fukuoka-shi, Fukuoka, 811-0295, Japan
2 College of Engineering, Nihon University, 1 Nakagawara, Tokusada, Tamuramachi, Koriyama, Fuku-shima Prefecture, 963-8642, Japan
3 Department of Materials Science, Faculty of Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu-shi, Fukuoka, 804-8550, Japan
4 Department of Mathematical Science and Electrical-Electronic-Computer Engineering, Graduate School of Engineering Science, Akita University, 1-1 Tegatagakuen-machi, Akita 010-8502, Japan
E-mail: tashima@fit.ac.jp (Daisuke TASHIMA)
ABSTRACT
Japanese distilled liquor (Shochu) is a widely produced alcoholic beverage in the Kyushu region of Japan. Our research group is developing inexpensive electrode material for supercapacitors (SCs) using waste generated during shochu production. In a previous study, shochu waste was carbonized and activated in an electric furnace; however, this process involved long production times and high running costs. To address these limitations, this study investigated the use of microwave activation as a faster and more cost-effective alternative. Activated carbon (AC) was fabricated from carbonized shochu waste mixed with potassium carbonate in a 1:3 weight ratio. Microwave power was set to 800 W, and the activation time was varied. The performance of the AC samples was evaluated by analyzing surface morphology, specific surface area, and pore volume, along with measurements of specific capacitance and charge-transfer resistance. The results revealed a maximum specific surface area of 1,063 m2/g and a specific capacitance of 229 F/g in 0.5 mol/L KOH at an activation time of 9 minutes. Additionally, microwave activation reduced power consumption costs by approximately 61.9% compared to activation using an electric furnace.
Keywords: Supercapacitor; shochu waste, activated carbon, microwave activation
1 INTRODUCTION
In recent years, global energy consumption has steadily increased, with fossil fuels accounting for a large proportion of total energy consumed. The close correlation between energy consumption and economic growth suggests that energy consumption will continue to rise with the economic development of emerging countries in Asia and beyond, raising concerns about the associated increase in carbon dioxide (CO2) emissions owing to massive fossil fuel consumption. Accordingly, many developed countries have set carbon-neutral goals, which are expected to drive greater adoption of renewable energy in the future. Nevertheless, the amount of electricity generated by renewable energy sources, such as wind and solar power is inherently unstable because of its dependence on land and weather. To stabilize this intermittent output, efficient energy storage devices are required. SCs have attracted attention as promising energy-storage devices. A SCs is classified as a capacitor and stores energy through the adsorption and desorption of ions [1]. While rechargeable batteries store electric charge generated by chemical reactions, the electrode material of a SCs is AC, a porous carbon material that adsorbs ions onto its surface when immersed in an electrolyte, forming an electric double layer that stores charge [2]. Compared with rechargeable batteries, SCs offer superior power density, but their energy density is lower [3, 4]. To increase the energy density of supercapacitors, research is also being conducted into improving capacitance through plasma treatment of activated carbon and increasing the voltage resistance of electrolytes [5, 6]. Currently, research on creating high-performance electrodes for SCs from biomass materials is attracting attention [7]. Examples include research using wheat husks [8], rice husks, and walnut shells [7]. The use of biomass-based polybenzoxazine aerogels [9] as raw materials is also being explored, as well as hybrid capacitors [10] and solid electrolyte capacitors [11]. Our group has explored the use of waste from shochu production, which is consumed in large quantities in the Kyushu region of Japan, as a material with great potential for use as an electrode material in SCs. AC generally contains three types of pores: macropores (> 50 nm), mesopores (2–50 nm), and micropores (< 2 nm), and often has a specific surface area of 1,500 m2/g to 2,500 m2/g. This pore structure is ideal for SC applications because it increases the contact area between the electrode and electrolyte, thereby enhancing capacitance. [12].
In a previous study, shochu waste as depicted in Figure 1 was carbonized and activated in an electric furnace [13]. However, this process is characterized by long production times and high running costs. Therefore, we investigated the use of microwaves to produce AC in less time and at a lower cost. Microwave activation differs from external heating methods (such as those using electric furnaces) in that it heats the material directly using microwaves without requiring an external heat source, offering the advantage of rapid heating [14-17].
In this study, AC was produced from carbonized shochu waste mixed with potassium carbonate (K2CO3) using microwave activation, and its surface area, pore volume, capacitance, and internal resistance in SCs setup were evaluated.
Figure 1. Shochu waste generated during the production of shochu.
2 EXPERIMENTAL METHODS
2.1 Preparation of AC
Figure 2 illustrates the production process of AC. Shochu waste was carbonized in an electric furnace under a nitrogen gas flow of 0.7 L/min at 600°C at a heating rate of 5°C/min for 1 hour. The carbonized shochu waste and K2CO3 solution (8 mol/L) were placed in a heat-resistant container and activated in a microwave oven under an atmosphere of 0.7 L/min N2 gas. Microwave activation was performed using a carbon-to-K2CO3 weight ratio of 1:3, a power of 800 W, and six different activation times (5, 6, 7, 8, 9, and 10 minutes). The samples were named MW5, MW6, MW7, MW8, MW9, and MW10, respectively. To extract K2CO3, AC was treated with 1 mol/L hydrochloric acid and washed with ultrapure water until the pH reached 7.0. The washed AC was then dried in an incubator at 100°C for 1 day.
Figure 2. Process of producing AC from shochu waste. The electric furnace and microwave were used for carbonization and activation, respectively.
2.2 Surface imaging
The surface of the AC was observed using a field-emission scanning electron microscope (JEOL, JSM-7100F) at two magnifications: 10,000× and 30,000×. Only the samples prepared with an activation time of 9 minutes were characterized.
2.3 Material characterization
The N2 adsorption isotherm was measured at 77 K to estimate the pore surface area and pore volume. The surface area was measured using the Brunauer–Emmett–Teller (BET) method [18] with a surface area/pore distribution analyzer (MicrotracBEL, BELSORP MINI II). The mesopore volume was calculated using the Barrett–Joyner–Halenda (BJH) method [19], and the micropore volume was calculated using the micropore analysis (MP) method [20-22].
2.4 Electrode preparation
The prepared AC was used to fabricate SCs electrodes, and their electrochemical performance was evaluated. Figure 3 illustrates the fabrication process of the AC electrodes. Electrodes with a diameter of 10 mm and a weight of 17.5 mg were prepared in a mold by mixing shochu waste derived AC, Ketjenblack (Lion, EC600JD), and polytetrafluoroethylene (TeflonTM30B, Polysciences Inc.) in a ratio of 8:1:1. The mixture was placed on a nickel mesh (Nilaco, NK1020S) and hot-pressed using an AH-200 machine (As One Corp. Japan) at 130°C and 10 MPa.
Figure 3. Electrode fabrication for electrochemical measurements.
2.5 Electrochemical measurements
The specific capacitance and electrochemical impedance of the AC were measured using an electrochemical measurement system (Meiden Hokuto Japan, HZ-5000, and NF Circuit Design Block Japan, FRA5022), as shown in Figure 4. Cyclic voltammetry (CV) was performed at a voltage of −0.9 to 0.1 V, with a sweep rate of 10 mV/s, a sampling interval of 1 s, and 10 cycles. An Ag/AgCl electrode was used as the reference electrode, and a platinum wire served as the counter electrode. The current density and potential values from the 10th cycle were used to calculate capacitance. Potassium hydroxide (KOH, 0.5 mol/L) was used as the electrolyte. Three samples were tested, and the average capacitance was calculated using Equation (1):
where C is the specific capacitance (F/g), I is the current (A),
The solution resistance and charge transfer resistances were measured using the electrochemical impedance method with a frequency-response analyzer. A bias voltage of 10 mV was applied in the frequency range of 10–20 kHz.
Figure 4. Schematic diagram of electrochemical measurements.
3 RESULTS AND DISCUSSION
3.1 Surface observations
Figure 5 shows surface images of the AC prepared with an activation time of 9 minutes, taken at magnifications of 10,000× (image (a)) and 30,000× (image (b)). Numerous macropores (larger than 50 nm) are observed, whereas mesopores or micropores are difficult to identify.
Figure 5. Surface images of AC prepared in 9 min activation, captured at magnifications of (a) 10,000× and (b) 30,000×.
3.2 Material characterization
Figure 6 (a-c) shows the nitrogen adsorption isotherms, micropore distributions, and mesopore distributions, respectively. The vertical axes in Figures 6(b) and 6(c) show the area distributions (dVp/ddp). The highest peak was observed for MW9, indicating the development of micropores of approximately 0.8 nm in diameter. Table 1 summarizes the specific surface area, mesopore volume, and micropore volume of each sample, calculated using BET, BJH, and MP methods. The highest-performing sample was MW9, with a specific surface area of 1,063 m2/g, a mesopore volume of 0.404 cm3/g, and micropore volume of 0.543 cm3/g. The specific surface area increased during the 5–9 minutes activation period and decreased from 9–10 minutes. A similar trend was observed for the mesopores and micropores, suggesting that the specific surface area increased owing to micropore development and decreased after 9 minutes owing to collapse of mesopores and micropores. In general, activation occurs at temperatures above 600°C. The presumed activation reactions when KOH is used as the activating agent are outlined below [23]:
At temperatures between 300-500 °C, the following reactions occur:
2KOH → K2O + H2O (2)
C + H2O → H2 + CO (3)
CO + H2O → H2 + CO2 (4)
Between 500-600 °C:
K2O + CO2 → K2CO3 (5)
At elevated temperatures of 600-800 °C:
K2O + H2 → 2K + H2O (6)
K2O + C → 2K + CO (7)
K2CO3 + 2C → 2K + 3CO (8)
Among these, reactions (7) and (8) are particularly important because they promote carbon gasification and development of pore structures. Furthermore, the metallic potassium generated during these reactions is believed to intercalate into carbon layers, thereby increasing the surface area. At higher temperatures, particularly around 900 °C, these reactions are significantly accelerated, leading to the formation of micropores. Based on these mechanisms, it is hypothesized that in activation processes involving rapid heating, such as microwave-assisted activation, K₂CO₃ may facilitate more effective pore development than KOH. The assumed chemical reaction between the carbonized material and K2CO3 is given by Equation (8). In this study, the optimum activation was reached at 9 minutes. If the reaction in Equation (8) occurs, the specific capacitance increases because of the development of pores caused by the carbon gasification and micropore expansion caused by metallic potassium intercalation. The decrease in specific surface area after 9 minutes is attributed to micropore collapse caused by excessive intercalation, as shown in Table 1.
Figure 6. Surface properties of AC prepared using different activation times. (a) Nitro-gen adsorption isotherm at 77K. (b) Micropore size distribution. (c) Mesopore size distribution.
Table 1. The surface area and pore volume obtained using various activation times.
| Sample name |
Activation Time (min) |
Specific Surface Area (m2/g) |
Mesopore Volume (cm3/g) |
Micropore Volume (cm3/g) |
|---|---|---|---|---|
| MW5 | 5 | 196 | 0.101 | 0.0980 |
| MW6 | 6 | 379 | 0.122 | 0.180 |
| MW7 | 7 | 692 | 0.220 | 0.338 |
| MW8 | 8 | 830 | 0.294 | 0.412 |
| MW9 | 9 | 1063 | 0.404 | 0.543 |
| MW10 | 10 | 566 | 0.188 | 0.277 |
3.3 Electrochemical measurements
Figure 7 (a) and (b) show the cyclic voltammograms and Cole–Cole plots of each sample. Table 2 lists the calculated specific capacitance, solution resistance, and charge-transfer resistance. The best performance, with a specific capacitance of 229 F/g, was confirmed for the sample with an activation time of 9 minutes. The specific capacitance tended to increase from the 5th to 9th minute and decreased between the 9th and 10th minute. This trend is similar to that of the specific surface area, indicating a proportional relationship between the specific capacitance and specific surface area. Using aqueous KOH as the electrolyte in this study had the disadvantage of a narrower voltage window compared with organic electrolytes but had the advantage of smaller ion diameters. The diameter of potassium ions in KOH is approximately 0.3 nm [24]. The presence of micropores larger than the ion diameter increases ion mobility. The samples obtained in this experiment were expected to develop micropores larger than 0.3 nm, allowing electrolyte penetration into the pores and increasing specific capacitance. We concluded that the best performance was obtained after 9 minutes activation by microwave, resulting in largest mesopore and micropore volumes. From these results, it appears that there is no relationship between solution resistance and charge-transfer resistance with respect to the microwave activation time. However, MW9, which had a large specific surface area and high capacitance, also exhibited relatively low solution resistance and charge transfer resistance. Table 3 compares the results of this study with previously reported data. Several reports have described the production of AC from biomass materials using methods other than microwaves, in which AC was evaluated as an electrode for SCs. For example, Wang et al. used wheat husks as raw material and prepared AC in a tubular carbonizer under a nitrogen atmosphere, obtaining a specific surface area of 2721 m2/g and a capacitance of 402 F/g [8]. Taurbekov et al. used rice husks and walnut shells, obtaining specific capacitances of 157.8 F/g and 152 F/g, respectively [7]. Periyasamy et al. used biomass-based polybenzoxazine aerogels, reporting a capacitance of 151 F/g [9]. Kokkiligadda et al. used DNA fibers and obtained a very high capacitance of 563 F/g [11]. An example of AC production from biomass materials using microwaves is as follows. He et al. prepared AC using microwave-assisted heat treatment of petroleum coke with KOH as the activation agent and reported a specific surface area of 2312 m2/g and a specific capacitance of 342.8 F/g after microwave-activation for 35 minutes, and a specific surface area of 1387 m2/g and a specific capacitance of approximately 230 F/g after 7 minutes of treatment at 700 W [14, 15]. Wu et al. prepared AC from peanut shells by KOH activation under microwave heating, reporting a specific surface area of 1277 m2/g and capacitance of 243 F/g after 7 minutes of microwave activation at 600 W [16]. These results show that using an H2SO4 electrolyte with a small ionic diameter can achieve high capacitance even with a smaller specific surface area. They also confirm that a higher concentration of KOH electrolyte results in larger capacitance. In our study, a relatively high capacitance was obtained with a low KOH concentration, which caused less damage to the electrode material and is expected to be advantageous for long-term reliability.
Figure 7. Electrochemical properties of AC prepared for different activation times. (a) Cyclic voltammograms; (b) Cole–Cole plots.
Table 2. Capacitance and internal resistance for different activation times.
| Sample name |
Activation Time (min) |
Specific Capacitance (F/g) |
Solution Resistance (Ω) |
Charge Transfer Resistance (Ω) |
|---|---|---|---|---|
| MW5 | 5 | 106 | 1.24 | 6.66 |
| MW6 | 6 | 157 | 1.20 | 2.93 |
| MW7 | 7 | 199 | 1.27 | 5.92 |
| MW8 | 8 | 209 | 1.84 | 5.16 |
| MW9 | 9 | 229 | 1.25 | 4.18 |
| MW10 | 10 | 188 | 1.47 | 4.91 |
Table 3. Comparison between this research and previously reported data.
| Material |
BET Surface Area (m2/g) |
Activation Agent |
Specific Capacitance (F/g) |
Electrolyte | Reference |
|---|---|---|---|---|---|
| Shochu waste | 1063 | K2CO3 | 229 | 0.5M KOH | This work |
| Wheat husk | 2721 | KOH | 402 | 1M H2SO4 | [8] |
| Rice husk | 1400 | KOH | 158 | 1M H2SO4 | [7] |
| Walnut shells | 2800 | KOH | 152 | 1M H2SO4 | [7] |
| Biomass-based polybenzoxazine aerogels | 673 | - | 151 | 1M H2SO4 | [9] |
| DNA fibers | - | - | 563 | 1M KOH | [11] |
| Petroleum coke | 2312 | KOH | 343 | 6M KOH/LiOH | [14] |
| Petroleum coke | 1387 | KOH | 230 | 6M KOH/LiOH | [15] |
| Peanuts shells | 1277 | KOH | 243 | 6M KOH | [16] |
3.4 Comparison of performance and power cost with previous studies
Figure 8 compares the specific surface area and specific capacitance obtained in this study with those from previous studies [25], and Figure 9 compares the power consumption (PC) per unit weight. In our previous study, the sample was activated under the same conditions as in the present experiment with microwave power (MWP) of 700 W and an activation time of 11 minutes. The sample exhibited a specific surface area of 856 m2/g, and a specific capacitance of 185 F/g. Another sample activated at 800°C using an electric furnace power (EFP) of 1.8 kW, with a heating/cooling rate of 5°C/min, and carbonized material-to-K2CO3 weight ratio of 1:3, also showed a specific surface area of 1063 m2/g and specific capacitance of 229 F/g. In both cases, the raw material was the same shochu waste used in this study. As shown in Figure 8, the specific surface area and specific capacitance in this study were 207 m2/g and 44 F/g higher, respectively, than those achieved using MWP at 700 W, but 350 m2/g and 91 F/g lower than those achieved using EFP. According to Figure 9, the PC for producing 1 g of AC using MWP at 800 W and an activation time of 9 minutes was 120 kWh/kg. This compares with 128 kWh/kg for 1 g of AC produced using the MWP at 700 W and an activation time of 11 minutes, and 315 kWh/kg for 5 g of AC produced using EFP at 1.8 kW and an activation time of 360 minutes. In other words, this study reduced the power cost by approximately 6.25% compared with MWP at 700 W and by 61.9% compared with the electric furnace method. One factor contributing to the good performance of electric furnace activation was that unlike microwave activation, the heating was external and conducted through thermal conduction. This strong temperature dependence allowed pores to form via the gasification of carbon with K2CO3 during the long activation process, making micropores less susceptible to collapse from metallic potassium intercalation compared with microwave activation. However, microwave activation offers the advantage of significantly lower power consumption, and its performance can be further improved through future innovations.
Figure 8. Comparison of specific surface area and specific capacitance measure in this study in previous studies. The values of the highest-performing samples are shown for each study.
Figure 9. Comparison of power consumption per unit weight measured in this study and in previous studies. This figure shows the power consumption for 9 min at 800 W of microwave power, 11 min at 700 W of microwave power, 1.8 kW of electric furnace power, and activation at 800°C, with a temperature rise and fall time of 5°C/min.
4 CONCLUSIONS
In this study, we prepared AC via microwave irradiation of Japanese distilled liquor waste carbon and K2CO3 at a weight ratio of 1:3, an output of 800 W, and an activation time of 9 minutes. The AC exhibited a maximum specific surface area of 1,063 m2/g and, when used as SCs electrode, achieved a specific capacitance of 229 F/g in 0.5 mol/L KOH. The optimum activation conditions were reached at 9 minutes, during which micropores were formed and the specific surface area increased owing to the gasification of the carbonized material via a chemical reaction with K2CO3 and the intercalation of the generated potassium atoms between carbon layers. Electrolyte penetration into the pores further enhanced the specific capacitance during the electrochemical measurements. The highest performance was obtained at 9 minutes, corresponding to the largest mesopore and micropore volumes. Beyond this point, the specific surface area decreased due to pore collapse. Compared with previous work, our process exhibited slightly lower performance but significantly reduced production time and operational cost. Therefore, pore development can be effectively achieved by microwave activation at 800 W for 9 minutes, which represents the most favorable condition.
ACKNOWLEDGEMENT
We express our gratitude to Kazuaki Harada, who works at the University of Kitakyushu, for his cooperation in conducting the research. This study was funded by the JSPS KAKENHI (grant number: JP24K00876).
REREFENCES
[1] Berrueta, A.; Ursúa, A.; Martín, I. S.; Eftekhari, A.; Sanchis, P., Supercapacitors: electrical characteristics, modeling, applications, and future trends. IEEE Access, 7, 50869 (2019).
[2] Şahin, M.E.; Blaabjerg, F.; Sangwongwanich, A., A comprehensive review on supercapacitor applications and developments. Energies, 15 (3), 674 (2022).
[3] Aziz, S.B.; Brza, M.A.; Mishra, K.; Hamsan, M.H.; Karim, W.O.; Abdullah, R.M.; Kadir, M.F.Z.; Abdulwahid, R.T., Fabrication of high performance energy storage EDLC device from proton conducting methylcellulose: dextran polymer blend electrolytes. J. Mater. Res. Tech., 9 (2), 1137-1150 (2020).
[4] Palisoc, S.; Dungo, J.M.; Natividad, M. Low-cost supercapacitor based on multi-walled carbon nanotubes and activated carbon derived from Moringa Oleifera fruit shells. Heliyon, 6 (1), e03202 (2020).
[5] Tashima, D.; Hirano, M.; Kitazaki, S.; Eguchi, T.; Kumagai, S., Solution-plasma treatment of activated carbon from shochu distillery waste for electrochemical capacitors. Mater. Chem. Phys., 254, 123523 (2020).
[6] Tomi, R.; Daisuke, T.; Toshihiko, K., Characteristics of electric double-layer capacitors based on solid polymer electrolyte composed of sodium polyacrylate. J. Phy.: Conference Series, 2368 (1), 012002 (2022).
[7] Wang, Y.; Chen, Y.; Zhao, H.; Li, L.; Ju, D.; Wang, C.; An, B., Biomass-derived porous carbon with a good balance between high specific surface area and mesopore volume for supercapacitors. Nanomaterials, 12 (21), 3804 (2022).
[8] Taurbekov, A.; Abdisattar, A.; Atamanov, M.; Yeleuov, M.; Daulbayev, C.; Askaruly, K.; Kaidar, B.; Mansurov, Z.; Castro-Gutierrez, J.; Celzard, A.; et al., Biomass derived High porous carbon via CO2 activation for supercapacitor electrodes. J. Compos. Sci., 7 (10), 444 (2023).
[9] Periyasamy, T.; Asrafali, S. P.; Lee, J., High-performance supercapacitor electrodes from fully biomass-based polybenzoxazine aerogels with porous carbon structure. Gels, 10 (7), 462 (2024).
[10] Shah, S. S., Biomass-derived carbon materials for advanced metal-ion hybrid supercapacitors: A step towards more sustainable energy. Batteries, 10 (5), 168 (2024).
[11] Kokkiligadda, S.; Ampasala, S.K.; Nam, Y.; Kim, J.; Bhang, S.H.; Um, S.H., Porous carbon electrode made of biomass DNAs for high-efficiency quasi-solid-state supercapacitor. Nanomaterials, 15 (4), 304 (2025).
[12] Dujearic-Stephane, K.; Gupta, M.; Kumar, A.; Sharma, V.; Pandit, S.; Bocchetta, P.; Kumar, Y., The effect of modifications of activated carbon materials on the capacitive performance: Surface, microstructure, and wettability. J. Compos. Sci., 5 (3), 66 (2021).
[13] Eguchi, T.; Tashima, D.; Fukuma, M.; Kumagai, S. Activated carbon derived from Japanese distilled liquor waste: Application as the electrode active material of electric double-layer capacitors. J. Clean. Produc., 259, 120822 (2020).
[14] He, X.; Geng, Y.; Qiu, J.; Zheng, M.; Long, S.; Zhang, X. Effect of activation time on the properties of activated carbons prepared by microwave-assisted activation for electric double layer capacitors. Carbon, 48 (5), 1662-1669 (2010).
[15] He, X.; Wang, T.; Qiu, J.; Zhang, X.; Wang, X.; Zheng, M., Effect of microwave-treatment time on the properties of activated carbons for electrochemical capacitors. New Carbon Materials, 26 (4), 313-319 (2011).
[16] Wu, M.; Li, R.; He, X.; Zhang, H.; Sui, W.; Tan, M., Microwave-assisted preparation of peanut shell-based activated carbons and their use in electrochemical capacitors. New Carbon Materials, 30 (1), 86-91 (2015).
[17] Zhou, F.; Liu, Q.; Gu, J.; Zhang, W.; Zhang, D., Microwave-assisted anchoring of flowerlike Co(OH)2 nanosheets on activated carbon to prepare hybrid electrodes for high-rate electrochemical capacitors. Electrochimica Acta, 170, 328-336 (2015).
[18] Brunauer, S.; Skalny, J.; Bodor, E.E., Adsorption on nonporous solids. J. Coll. Interf. Sci., 30 (4), 546-552 (1969).
[19] Barrett, E.P.; Joyner, L.G.; Halenda, P.P. The determination of pore volume and area distributions in porous substances. I. computations from nitrogen isotherms. J. Amer. Chem. Soc., 73 (1), 373-380 (1951).
[20] de Boer, J.H.; Lippens, B.C.; Linsen, B.G.; Broekhoff, J.C.P.; van den Heuvel, A.; Osinga, T.J., The t-curve of multimolecular N2-adsorption. J. Coll. Interf. Sci., 21 (4), 405-414 (1966).
[21] Sing, K.S.W. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984). Pure and Appl. Chem., 57 (4), 603-619 (1985).
[22] Sing, K.S.W. The use of physisorption for the characterization of microporous carbons. Carbon, 27 (1), 5-11 (1989).
[23] Ikuo Abe, Production methods of activated carbon, TANSO, 225, 373-381 (2006)(in Japanese)
[24] Degrève, L.; Vechi, S.M.; Junior, C.Q., The hydration structure of the Na+ and K+ ions and the selectivity of their ionic channels. Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1274 (3), 149-156 (1996).
[25] Izaki, A.; Tashima, D.; Kawabata, T., Making Shochu waste activated carbons by microwaves activation and performance evaluation, The 74th Joint Conference of Electrical, Electronics and Information Engineers in Kyusyu, 08-2A-03, 262 (2021).
ACKNOWLEDGEMENT
We express our gratitude to Kazuaki Harada, who works at the University of Kitakyushu, for his cooperation in conducting the research. This study was funded by the JSPS KAKENHI (grant number: JP24K00876).
